The use of dielectric materials to increase capacitance is well known and long-used. Earlier capacitor dielectrics fell into two categories. The first category of dielectrics has a relatively temperature-independent dielectric constant but the value of the dielectric constant is low, e.g., 5-10. Materials such as electrical porcelain and mica fall in this category. The second category of dielectrics has very high dielectric constant, e.g., 1000 or more, but they are quite temperature dependent. An example is barium titanate, BaTiO3.
Since the capacitance is proportional to the dielectric constant, high dielectric constant materials are desired. In order to perform acceptably in tuning or resonance circuits the dielectric must have a dielectric constant that exhibits minimal temperature dependence; otherwise small changes in ambient temperature throw the circuit out of resonance. Other applications require a dielectric constant that exhibits minimal frequency dependence. It is also desirable to have the loss or dissipation factor as small as possible.
For many microwave devices the important material features are the dielectric tunability, i. e., the change in dielectric constant with applied voltage, and low dielectric loss. Barium strontium titanate, Ba1−xSrxTiO3, has been used in some such applications but the need persists for materials with better properties.
Deschanvres et al., Bull. Soc. Chim. Fr. 4077 (1967) report the preparation of CaCu3Ti4O12 with the perovskite structure and a lattice constant of 0.7393 nm.
Bochu et al., J. Solid State Chem. 29, 291 (1979) disclose the synthesis and structure of CaCu3Ti4O12 and related titanates and report the lattice constant to be 0.7391 nm.
Yandrofski et al., U.S. Pat. No. 5,472,935, disclose tunable microwave and millimeter wave devices incorporating tunable ferroelectrics.